The invention relates generally to optical data storage techniques and more particularly to holographic data storage techniques.
Holographic storage is the storage of data in the form of holograms, which are images of three dimensional interference patterns created by the intersection of two beams of light, in a photosensitive storage medium. Both page-based holographic techniques and bit-wise holographic techniques have been pursued. In page-based holographic data storage, a signal beam which contains digitally encoded data, typically a plurality of bits, is superposed on a reference beam within the volume of the storage medium resulting in a chemical reaction which, for example, changes or modulates the refractive index of the medium within the volume. This modulation serves to record both the intensity and phase information from the signal. Each bit is therefore generally stored as a part of the interference pattern. The hologram can later be retrieved by exposing the storage medium to the reference beam alone, which interacts with the stored holographic data to generate a reconstructed signal beam proportional to the initial signal beam used to store the holographic image. In bit-wise holography or microholographic data storage, every bit is written as a microhologram or reflection grating typically generated by two counter propagating focused recording beams. The data is then retrieved by using a read beam to diffract off the microhologram to reconstruct the recording beam.
Recently, dye-doped data storage materials based on polymeric materials have been developed for holographic data storage. The dyes have a narrow absorption band at visible light wavelengths. Upon light absorption, they undergo a photochemical conversion, which produces a change of refractive index of the material, according to the Kramers-Kronig relation. Due to the resonant absorption of the dyes, the refractive index change could be high (˜0.01). This provides a beneficial potential for obtaining a high data capacity. In addition, the thermoplastic material has a much smaller shrinkage compared with photopolymer material and has very good optical quality, and is comparatively economical. These features make the dye-doped thermoplastics a very attractive candidate for holographic storage.
As discussed above, data is typically written by focusing and interfering two laser beams within the media to photochemically convert specific regions. Unfortunately, the light must pass through the entire media even though only specific areas are to be converted. Although the beams are most intense in the focused areas and produce the most conversion, low levels of the dye are converted throughout the media. Thus, after one layer of data is written and another layer is then written into subsequent layers, an undesirable additional photochemical conversion of the data in the first layer occurs. This ultimately limits the number of layers of data that can be written into the media, which limits the overall storage capacity. A second problem arises from the fact that the material should have high QEs in order to have a useful sensitivity for commercial applications. Materials with high QEs are then subject to rapid photochemical conversion of the data even when using a low power reading laser, so the data can only be read a limited number of times before the disk becomes unreadable.
Accordingly, a technique is needed to address one or more of the foregoing problems in holographic data storage devices.